Resonant circuits are used in many electrical devices where it is desired to inductively transfer energy across an air gap, for example in induction heaters or cooktops, induction heat treatment, induction furnaces and inductive power transfer devices such as those used for charging batteries or other energy storage devices in electric vehicles, cell phones, portable computers, or other rechargeable appliances. Resonant circuits are also used in other devices including lighting circuits, transformers and motors.
This use of what is termed a “loaded resonant circuit” is characterised by a circuit having a resonant build-up of energy in an inductive component of the circuit and the transfer of a portion of this energy across space by magnetic flux, which is inductively coupled or linked into an inductive component of a load circuit. High energy transfer efficiencies can be achieved in well designed systems because only a small fraction of the resonant energy is being consumed relative to the large reactive energy oscillating in the resonant circuit. However, the circuit energy transfer efficiencies decrease as the distance between the inductively coupled or linked inductive components increases.
Conventional induction heating drive circuits include a number of inverter configurations, the most common being quasi-resonant, half bridge, and full bridge inverter drives. These are characterised by the direct inputting of energy into the load circuit over half of every half cycle or during every second half cycle. These standard inverter topologies are typically used in other inductive power transfer devices as well.